Breather Level: The Machine, ironically. It's big and labyrinthine, but there are only two places to die (the twin drains at the bottom), so you can more or less relax and take your time. The Veg-O-Machine itself also turns out to be a Breather Boss... followed right thereafter by a Difficulty Spike up the Ro-butt-nik called "The Showdown".
Ear Worm: Given the difficulty of the game, Toxic Caves might turn into this since it's reasonable that some players may have spent a lot of time here.
Funny Moments: One of the "did you know" facts in this game's "Museum" section in Sonic's Ultimate Genesis Collection is unusually snarky: "A Pinball Defense System, Robotnik? That was your big idea to stop Sonic? Really?"
Nightmare Fuel: Whether intentional or not, this whole game has given many a player nightmares, particularly for the dark level designs, difficulty level, and some rather... frightening sound effects.
Porting Disaster: The Game Gear/Master System version. Although the levels are remarkably similar compared to other Sonic the Hedgehog ports (not necessarily by its own merits), whatever physics existed in the Genesis version were thrown completely out the window. Worse is the platforming engine, in which Sonic has an innate tendency to get himself stuck.
Scrappy Mechanic: The steam valves in Lava Powerhouse used to get into the air so Sonic can bounce off of randomly spawning Clucks, and rise up to higher levels. They're very finicky while presenting no actual challenge or danger.